There are a few engines that continually impress with their reliability, and among them is the Mercedes-Benz OM617. It hails from automaker's back-catalog, and it's a great example of a reliable diesel engine that just will not quit. The OM617 is a little unusual in the fact that it's a 3.0-liter inline-five diesel engine, and impressively, it enjoyed a career at Mercedes that stretched from the mid-1970s, right into the 1990s. Four-cylinder engines weren't cutting it, and six-cylinder lumps would pose fitment issues due to the increase in size, so five cylinders made perfect sense. It's important to note that Mercedes chose another cylinder for more grunt, as opposed to stressing a four-cylinder engine out with performance modifications, as therein lies the secret to its remarkable reliability.